Qingming Zhu

Ultra-compact tunable silicon nanobeam cavity with an energy-efficient graphene micro-heater

We propose and experimentally demonstrate an ultra-compact silicon photonic crystal nanobeam (PCN) cavity with an energy-efficient graphene micro-heater. Owing to the PCN cavity with an ultra-small optical mode volume of 0.145 µm3, the light-matter interaction is greatly enhanced and the thermo-optic (TO) tuning efficiency is increased. The TO tuning efficiency is measured to be as high as 1.5 nm/mW, which can be further increased to 3.75 nm/mW based on numerical simulations with an optimized structure. The time constants with a rise time constant of τrise = 1.11 μs and a fall time constant of τfall = 1.47 μs are obtained in the experiment.

SQNR Improvement Enabled by Nonuniform DAC Output Levels for IM-DD OFDM Systems

We propose and demonstrate via both simulation and experiment a scheme for a low-bit resolution digital-to-analog convertor (DAC) to improve the signal-to-quantification noise ratio (SQNR) of an intensity-modulated direct-detection (IM-DD) orthogonal frequency division multiplexing (OFDM) signal. In this scheme, the discrete output levels of a DAC are nonuniform and properly assigned based on the distribution of the OFDM signal. Such nonuniformity allows a better exploitation of output levels for the OFDM signal so that low-level waveforms that appear much more frequently experience mitigated quantization noise. To calculate these optimized output levels with the distribution of an OFDM signal, a numerical algorithm is proposed and verified. Our proposed scheme is proven effective in the simulations for Gaussian-distributed signals and brings over 3.6-dB and ∼2.5-dB improvements on SQNR and receiver sensitivity, respectively. The feasibility is also verified by a proof-of-concept experiment with ∼7.4-Gbps quadrature phase shift keying (QPSK)-modulated OFDM signals output by a 3-bit DAC. Our scheme enables transmission of such signals with no further optimization with ∼−25-dBm receiver sensitivity, while it is impossible using a conventional way. Both simulation and experimental results validate our scheme as a promising way to be implemented in low-bit resolution DACs for IM-DD OFDM applications.

Spectrally-efficient direct detection of QAM signal by orthogonal carrier interleaving

We propose and numerically demonstrate a scheme that enables the direct detection of QAM signals by a single photodetector. 40-Gb/s 16-QAM signal over 22-km fiber transmission is recovered without sacrificing optical spectral efficiency.

Efficient Out-of-Band Spectrum Suppression in WDM-OFDM Systems

We propose and experimentally demonstrate a new scheme for orthogonal frequency division multiplexing (OFDM) sidelobe suppression by inserting sidelobe suppression prefix (SSP) ahead of cyclic prefix (CP). The feasibility of the proposal is verified by both simulation and experiment.

Energy-efficient optical network units with simplified FFT operation in direct-detection OFDM PON

We propose and experimentally demonstrate simplified FFT operation to improve the energy efficiency of ONUs in OFDM PON. Our proposal can achieve an energy saving of 14.6% compared to the conventional OFDM PON.

PAPR Reduction and Nonlinear Impairment Mitigation by Frequency Guard Band-Tone Reservation for DD-OFDM

A novel OFDM PAPR reduction scheme by exploiting frequency guard band (FGB) is proposed and experimentally demonstrated. Effective mitigation of nonlinear impairments is validated and BER improvement of 2.6 dB is achieved over 50-km fiber transmission without sacrificing data rate.